Capillary column gas chromatography (CC-GC) is an important branch of chemical analysis. CC-GC is widely used as a sensitive technique for the analysis of trace components because using this technique it is possible to resolve a multiplicity of components contained in a sample in trace quantities. A wide variety of CC-GC methods and apparatuses have been developed.
A relatively large volume of the sample has to be injected to detect and determine components present in the sample at extremely low concentrations. However, with CC-GC the sample capacity is so limited that when a relatively large volume of the sample is injected into the injection port, detection of the components subject to analysis frequently becomes difficult or their determination becomes inaccurate because of the severe tailing of the solvent of the sample, which causes the peaks of the components of interest with relatively short retention times to overlap with those of the tailing solvent. This is caused by a part of the large quantity of the solvent in the sample flowing slowly into the capillary column over a considerable period of time after inflow of the components subject to analysis has taken place, in spite of the fact that ideally the vapors of the solvent and all the components of interest should flow into the column at about the same time.
The injection modes for capillary gas chromatography can be divided broadly into four categories; (1) the split mode; (2) the splitless mode, (3) the on-column mode and (4) the direct mode, as discussed in detail in the article "Sample Injection In Gas Chromatography" by Walter Jennings et al in Journal of Chromatographic Science, Vol. 24, January 1986, pp 34-40, which is herein fully incorporated by reference.
In the split mode generally a small volume of liquid sample, comprising a low boiling solvent and the sample components of interest, is flash vaporized in a chamber having a carrier gas flowing therethrough. The flow of vaporized sample in the carrier gas is then split so that, for example, about 99% of the flow is discarded and only about 1% enters the column. The split mode can provide good separation between the solvent and the sample components of interest, i.e.. little solvent "tailing", but at the expense of sensitivity of detection of the sample components of interest. It has long been needed in capillary gas chromatography to develop an injection mode having little solvent tailing and improved sensitivity.
In the splitless mode of injection, a relatively dilute sample is vaporized slowly (e.g., 30 seconds) into the column and then, after a suitable delay, the injection port is purged to prevent tailing by the solvent of the sample. Pre-concentration of the sample can be required for trace analysis because the maximum injectable amount of the sample is 1-2 microliters in this mode of injection. The splitless mode can provide good separation between the solvent and the sample components of interest, i.e., little solvent tailing, but as the expense of operational ruggedness, e.g., small differences in the way the sample is injected by a syringe can have profound qualitative and quantitative effects as discussed by Jennings et al, and a portion of the analytical components of interest can be lost due to the purging of the injection port. It has long been needed in capillary gas chromatography to develop an injection mode having little solvent tailing and a high degree of operational ruggedness.
In the on-column injection mode the sample is injected into the column. However, this mode of injection is generally thought to be limited to the injection of relatively small volumes of sample and other problems as discussed by Jennings et al which limitation also limits the sensitivity of detection of the sample components of interest.
In the direct injection mode, the sample is injected into a heated chamber and vaporized. The vaporized sample is then flowed into the column by the carrier gas without splitting. Capillary columns having internal diameters of more than 0.5 millimeter are less sensitive to injection problems and have been adapted to injectors designed for use with packed columns as shown by Jennings et al in FIG. 6 and in FIG. 7. The adaptions shown include the provision of a cylinder inserted into a packed column injector. The sample is vaporized in the cylinder and then entirely introduced into the column without splitting. This general approach has also been applied to conventional capillary columns having an internal diameter of less than 0.5 millimeter, where injection problems can be severe, as described in a book by Milton L. Lee et al entitled "Open Tubular Column Gas Chromatography" 1984, published by John Wiley & Sons, ISBN 0-471-88024-8, pp 116-118, herein fully incorporated by reference. Lee et al describe a sample vaporization cylinder of 0.7 millimeter internal diameter and states that "The goal of having minimum injector contribution to bandspreading can be achieved by using a narrow-bore vaporization chamber." The direct injection system described by Lee et al requires a slow injection of a sample volume greater than about 1 microliter to prevent undesirable backflushing of vaporized sample down the septum purge incorporated therein. Thus, the direct injection method for the injection of a relatively large volume of sample has been applicable only to capillary columns having an internal diameter of more than 0.5 millimeter to obtain good separations of the peak of the solvent and those of the substances subject to analysis in the sample, and it is virtually inapplicable to capillary columns of smaller inside diameter because of severe tailing.
It is, therefore, an object of the present invention to provide a chromatographic analytical apparatus and method most preferably applicable for permitting a relatively large sample injection volume onto a capillary column of relatively small internal diameter so that the components of interest in the sample may be sensitively detected, and with a significant reduction in the aforementioned problem of solvent tailing.